1. Field of the Invention
The present invention relates to a zoom lens and an imaging device, and more particularly to a zoom lens which is suitable to be mounted on a television camera or video camera and an imaging device having the zoom lens.
2. Description of the Related Art
In the related art, a zoom lens of four group constitution mounted on the television camera or video camera is well known. The zoom lens of four group constitution has a first group having a positive refractive power, a second group having a negative refractive power and becoming a variable power group by moving at the time of variable power, a third group having a positive or negative refractive power and correcting a variation in the image point position caused by the variable power, and a fourth group having a positive refractive power in order from an object side, for example.
In JP-B-7-1337, a zoom lens which comprises a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power in order from an object side, and achieves the variable power by moving the first group, the third group and the fourth group was described. The first group of the zoom lens as described in JP-B-7-1337 has a negative meniscus lens, a positive lens, a compound lens having a positive refractive power, and a positive lens which are arranged in order from the object side.
In JP-B-2711717, a zoom lens which comprises a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power in order from an object side, and achieves the variable power by moving all the four groups was described. The first group of the zoom lens as described in JP-B-2711717 has a negative meniscus lens, a positive lens, a compound lens having a positive refractive power, and a positive lens which are arranged in order from the object side.
In JP-B-3376171, a zoom lens which has a first group having a positive refractive power, a second group having a negative refractive power for variable power, a third group for correcting an image surface variation caused by the variable power, and a fourth group having an image formation operation in order from an object side was described. The first group of the zoom lens as described in JP-B-3376171 has a front group having a negative refractive power and a back group composed of a compound lens having a positive refractive power and a positive lens which are arranged in order from the object side.
In recent years, there is a demand that the zoom lens mounted on the television camera or video camera has a higher performance, and further is reduced in size as the camera has a smaller size. Also, there is a higher demand for the zoom lens having a high variable power ratio of 20 times or more as the needs are diversified.
The zoom lenses as described in JP-B-7-1337 and JP-B-2711717 have a variable power ratio as low as about 4 to 5 times, and can not meet the requirement of a high variable power ratio. Though the zoom lens having high variable power ratio was described in JP-B-3376171, there is a room for improvement in the respects of smaller size and higher performance.
Generally, the zoom lens comprises a variable power group taking charge of the variable power, and a first group disposed on an object side of the variable power group, in which the variable power is achieved by the variable power group which scales the focal length of this first group. The first group has great influence on the aberration or aberration due to a variation of the focal length, and this influence is more remarkable at the higher magnification, whereby it is required to correct the aberration for the zoom lens more strictly than the low power zoom lens. Accordingly, it is an important subject how the first group is constituted in the zoom lens.
On the other hand, since the focal length of each lens group is set to be shorter for the size reduction, the focal length of the first group is also set to be shorter. However, if the higher variable power ratio is obtained simply by making the focal length of each lens group shorter, a large aberration variation is caused by the variable power, whereby it is very difficult to obtain a high optical performance over the entire variable power range. Especially if the focal length of the first group is shorter, there was a problem that the residual secondary spectrum of longitudinal chromatic aberration abruptly increases near the telescopic end at the time of variable power.
In the zoom lens for television camera or video camera, if the variation of chromatic aberration is greater, the image quality of color image is extremely degraded. Therefore, in the zoom lens for these uses, it is required that the chromatic aberration is excellently corrected over the entire variable power range from the wide-angle end to the telescopic end.